I have a Casio EX-S12. It has a setting for image size, but also a separate setting for what it calls "image quality". There are three image quality values: "Fine", "Normal", and "Economy".

The Casio manual is not clear on what image "quality" actually means -- specifically how it's different from image size. All they say is that the "Fine" setting "helps to bring out details when shooting a finely detailed image of nature that includes dense tree branches or leaves, or an image of a complex pattern." I'm confused because isn't this what image size is all about (more pixels = more detail)?

In terms of memory usage, the manual also says that for a 5 megapixel picture, a "Fine" image takes up 2.99 MB, a "Normal" image takes up 1.62 MB, and an "Economy" image takes up 1.12 MB, so this "image quality" setting of theirs is certainly having a significant impact on memory usage.

My question is, what exactly is "image quality", if it's not image size? What is the "thing" that is taking up additional memory?

3 Answers
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Image size is what if often called resolution, basically the number of pixels stored in the image file. So on a 12 megapixel camera, you can usually choose between 12 MP, 6 MP and 3 MP or similar values.

Image quality is independent of size and is usually called compression. This controls how much information is discarded from images while they are saved.

You can read this article which I wrote several years ago for a comparison between the two.

Quality refers to the level of JPEG compression used. JPEG images use lossy compression to reduce filesizes. The compression method converts the image into a frequency representation (a set of waves instead of a set of pixels) and removes frequencies whose amplitude are below a certain threshold, on the assumption that missing these will affect image quality less.

Whilst this works well for mild compression, after a while you start loosing important components and thus detail in your image.

That's a nice distinction you make between pixel representation vs. frequency representation. Thank you for that.
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JackAug 4 '11 at 16:25

Matt, you actually put it the reversed (and wrong) way - the lossy part of JPEG discards the high frequencies, rather than the low ones. Additionally, the cut-off is not a strict frequency, but rather a decreasing series of coefficient bit depth (this is a quantization step determined by a quantization size matrix)
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ysapAug 4 '11 at 23:58

@ysap JPEG compression removes frequencies below a certain amplitude regardless of the actual frequency, it just happens that the ones you lose tend to be high frequencies. I'll make that clear in the answer.
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Matt GrumAug 5 '11 at 0:02

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I was simplifying, the question didn't seem to necessitate a full description of JPEG. But as I understand it the compressor is free to specify explicitly the full quantisation table (i.e. a separate coefficient for each frequency), even a fully degenerate one, but the point of JPEG is to remove frequencies with the minimal impact on visual quality, which is implicitly the ones with low amplitudes...
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Matt GrumAug 5 '11 at 0:22

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OK, doing some reading, it seems like the standard does give matrices, which are being used in many applications where the Quality parameter is merely a scaling factor on these matrices, but the encoder can generate optimized matrices based on the image. But anyway, removing frequencies based on amplitude below threshold is just the side effect. The real deal is that the quantization makes the coefficients smaller and thus representable in less number of bits. Those "unlucky" to be more than lsb, get rounded to zero and removed completely.
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ysapAug 5 '11 at 1:49

JPEG is a "lossy" compressed image format meaning that, at various levels of compression, data is thrown away to make the image smaller in file size as opposed to image dimensions. So "fine" setting on the quality is going to do little compression and therefore lessen the amount of data discarded as a result and give a nicer looking image. An "economy" setting is going to throw away more data, which will degrade the image detail, but give you a smaller file size.

In general, unless you have a real serious need to do it, I would be shooting at the largest image size and at the highest image quality.

Yes, I suppose it's better to shoot at the highest quality, and then I could always make a lower quality copy for particular purpose (e.g., emailing) if need be. Thank you.
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JackAug 4 '11 at 16:24

You almost never want to e-mail the full-size image anyway, regardless of quality setting. Better to resize to a size that is appropriate for how the image will be used.
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Michael KjörlingAug 5 '11 at 6:13